Thermophotovoltaic (TPV) devices convert thermal energy to electric power using the same principle of operation as solar cells. In particular, a heat source radiatively emits photons which are incident on a semiconductor TPV cell. Photons with an energy greater than the semiconductor bandgap (E.sub.g) excite electrons from the valence band to the conduction band (interband transition). The resultant electron-hole-pairs (ehp) are then collected by metal contacts and can power electrical loads. Photons with energy less than E.sub.g are parasitically absorbed as heat. In order to increase the efficiency of a TPV energy system some form of spectral control is employed to reflect the photons with energy below E.sub.g back to the emitter before they are parasitically absorbed. This invention proposes a TPV design concept that uniquely integrates the spectral control and cell into a single design.
Previous researchers have attempted to improve TPV conversion efficiency through spectral control by three different methods. In the first method researchers modified the emission spectrum of the thermal radiator in an attempt to suppress emission of below-bandgap energy. Several techniques have been tried including surface texturing and rare earth oxide coating. Nelson, U.S. Pat. No. 4,764,104 provides one example.
In the second method, researchers position selective filters in front of the TPV cell. These filters transmit most of the above-bandgap energy but reflect below-bandgap energy back to the radiator for "recycling". A publication by H. Kostlin and G. Frank; "Thin-film reflection Filters". Phillips Tech. Rev. 41 1983/84, No. 7/8 describes this technology.
In the third method, a highly reflective coating is applied to the back of the TPV cell. Most of the above-bandgap energy is absorbed in the active region of the cell while most of the below-bandgap energy passes through the cell reaching the back surface. There it is reflected and returned to the radiator after passing through the cell a second time. Another publication by R. M. Swanson; "Silicon Photovoltaic Cells in TPV Conversion"; EPRI Project 790-2; Interim Report ER-1272, December 1979 provides an example of this technique. One critical issue associated with this approach is the amount of below-bandgap energy parasitically absorbed during transit through the cell. This invention addresses that issue.